The present invention relates to a convenient method for producing slag ballast having favorable properties from molten slag obtained from blast furnaces and other smelting furnaces.
Molten slag as referred to above should be understood to include those by-products obtained in the metal smelting processes in blast furnaces, open hearth furnaces, converters, and electric furnaces, as well as in the metal melting processes in cupolas and the like. In the following description, molten blast furnace slag will be taken as a representative example and will be simply referred to as slag.
As is well known, in the pig iron production process in blast furnaces, molten slag as hot as 1400.degree.-1500.degree. C. is obtained in an amount as much as 300 kg per ton of pig iron. Such molten slag consists primarily of SiO.sub.2, Al.sub.2 O.sub.3, CaO, and MgO as can be seen from the following figures representing an example of the slag composition:
______________________________________ SiO.sub.2 AlO.sub.2 CaO MgO S MnO T.Fe others ______________________________________ 35.0 15.3 42.1 5.6 0.85 0.1 0.15 0.30 (%) ______________________________________
Normally, S is present in an amount of 0.8-1.2% in slag.
Recently, serious attempts have been made to find a way of utilizing molten slag. A representative utilization is so-called slag ballast which is obtained by crushing and sieving of slowly cooled and solidified molten slag. Because slag ballast possesses hydraulicity, it is mostly used as the subbase material in road construction.
Conventional processes of obtaining slag ballast normally include (1) pouring of molten slag from blast furnaces into ladle cars, (2) transportation of the molten slag in ladle cars to a cooling yard, (3) pouring the molten slag over a cooling yard, (4) natural cooling of the molten slag for 6 to 8 hours, (5) water cooling of the slag for 3 to 4 hours, (6) recovering the solidified slag by such means as bulldozers, (7) crushing, and (8) sieving.
However, such conventional processes are disadvantageous because (1) a large cooling yard is required to process large amounts of slag, (2) for the air and water cooling of slag, much time is consumed, and (3) much manpower is required to recover the cooled and solidified slag. Furthermore, slag ballast obtained through conventional methods as described above is not very satisfactory as road subbase material because it contains a substantial amount of sulfur.
Of late, a number of improved methods have been proposed to eliminate some of the shortcomings of slag ballast. For example, for the purpose of improving the compressive strength of slag ballast, proposals have been made to reduce the number of holes therein by mixing steel rolling scale, converter slag, and Indian ferromanganese ore into the molten slag. For the purpose of fixing the sulfur contained in slag and preventing it from dissolving in water, proposals have also been made to fix the sulfur in the slag as sulfides of iron or manganese which are not soluble in water by mixing converter slag, steel rolling scale, and Indian ferromanganese ore the molten slag.
However, to produce this improved slag ballast, not only are large yards required, long hours wasted, and much manpower required for recovering the solidified slag, but also for crushing the improved high density and hard slag containing additives such as described above, special crushing devices may be required because such improved slag is too hard to be crushed by conventional crushers. It may be mentioned that while ordinary slag ballast has a compressive strength of 20 to 24 kg/cm.sup.2, the above described improved slag ballast has a compressive strength of 25 to 35 kg/cm.sup.2.
On the other hand, in view of the growing general concern with energy saving, efforts are increasingly being made to recover and utilize conventionally wasted energy, but no attempts have been successful so far during recovering the heat of molten slag in the process of producing slag ballast from molten slag. The chief reason for the lack of satisfactory methods of recovering and utilizing heat during conventional slag ballast production processes is that the hot molten slag is very difficult to handle and leaving it to cool by itself has been the only technically feasible method of cooling it.